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Abstract

Here we demonstrate a new imaging system that addresses several major problems limiting the clinical utility of conventional adaptive optics scanning light ophthalmoscopy (AOSLO), including its small field of view (FOV), reliance on patient fixation for targeting imaging, and substantial post-processing time. We previously showed an efficient image based eye tracking method for real-time optical stabilization and image registration in AOSLO. However, in patients with poor fixation, eye motion causes the FOV to drift substantially, causing this approach to fail. We solve that problem here by tracking eye motion at multiple spatial scales simultaneously by optically and electronically integrating a wide FOV SLO (WFSLO) with an AOSLO. This multi-scale approach, implemented with fast tip/tilt mirrors, has a large stabilization range of ± 5.6°. Our method consists of three stages implemented in parallel: 1) coarse optical stabilization driven by a WFSLO image, 2) fine optical stabilization driven by an AOSLO image, and 3) sub-pixel digital registration of the AOSLO image. We evaluated system performance in normal eyes and diseased eyes with poor fixation. Residual image motion with incremental compensation after each stage was: 1) ~2–3 arc minutes, (arcmin) 2) ~0.5–0.8 arcmin and, 3) ~0.05–0.07 arcmin, for normal eyes. Performance in eyes with poor fixation was: 1) ~3–5 arcmin, 2) ~0.7–1.1 arcmin and 3) ~0.07–0.14 arcmin. We demonstrate that this system is capable of reducing image motion by a factor of ~400, on average. This new optical design provides additional benefits for clinical imaging, including a steering subsystem for AOSLO that can be guided by the WFSLO to target specific regions of interest such as retinal pathology and real-time averaging of registered images to eliminate image post-processing.

Figures (14)

Schematic diagram of the control blocks of the multi-scale stabilization engine. Solid red line denotes AOSLO signal path. Dashed dark red line is WFSLO signal path, thin blue line shows fixation path and thick gray line denotes pupil camera signal path. As can be seen here, WFSLO driven stabilization runs in open loop with the action of TTM2 not being ‘seen’ by the WFSLO system while TTM1 runs in closed loop where the AOSLO image captures the action of both TTMs.

A large FOV image is used as the reference frame for a narrow target image (brighter narrow image in foreground) for WFSLO eye tracking. A narrow target image increases the frame rate to ~51 fps during WFSLO driven optical stabilization. The narrow target image can be placed anywhere in the large reference image FOV, allowing target images to be optimized for cross-correlation by selecting an area with high contrast retinal features. Scale bar is 500μm.

Motion artifacts (a) appear in some stabilized images due to unsuccessful cross-correlation between the reference (b) and target (c) images. Yellow outline in (c) denotes area shown in (a). After a second round of cross-correlation, motion artifacts are cropped out, and the final image (d) is used for real-time averaging. Scale bars are 50 µm.

Integrated steering and stabilization allows a target area (dashed rectangle) in the WFSLO image (right) to be imaged with AOSLO (left) with minimal overlap between adjacent fields (e.g. areas as 1, 2, 3). Scale bar for AOSLO is 50 um and scale bar for WFSLO image is 500 um.

The resolution of the AOSLO was not compromised with the additional features we have added, as we maintained the visibility of not only the foveal cones (left) but also rods in the periphery (right). Scale bars are 20 µm (left) and 10 µm (right).

Each stage of motion compensation reduces the amplitude of residual motion. Note that each stage is shown running consecutively here for illustrative purposes but in practice all three stages run simultaneously. White squares in left image and center images denote areas of images shown to the right. Scale bars for the images are (from left to right): 25 µm, 5 µm, and 1 µm.